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Apparently some of those discs were the same galaxy see in multiple places due to the power of the lensing?

Yes, and they confirmed it was the same galaxy using spectroscopy: https://webbtelescope.org/contents/media/images/2022/035/01G...

Presumably the path lengths are probably not identical... That means we might be able to see the same galaxy twice at different points in time?

That should be a really good way to check our models are accurate!

This has actually been done before - scientists have been able to image the same supernova multiple times from a gravitationally lensed galaxy: https://www.universetoday.com/151581/astronomers-saw-the-sam...

Now that is fascinating.

These kinds of observations have frequently been made for lensed quasars. Quasars are fairly stochastic and have random bursts in their brightness. If you see multiple images of the same quasar, you can see one of the images get brighter and then some time later (usually of order days) you can see the other image get brighter.

By measuring the time delay and the separations between the images you can get an estimate of the Hubble constant.

Interesting, it should be theoretically possible to use gravitational lensing to view a past version of earth, no?

You would need a gravitational lens so powerful it not only bent light, but made the light perform a complete slingshot around it so that it would be aimed back at Earth. The nearest such object is the Milky Way's black hole, and there's no way we'd be able to resolve Earth in an image coming from a) that far away and b) that close to the galactic core where so much other stuff is in the way.

But could a photon make such a journey? Yes. But you'd never be able to recognize it against everything else.

To correct myself, because I didn't actually bother to check first, there are actually black holes or probable black holes closer than the center of the galaxy. Like, a lot closer. [1] And we've seen a lensing effect from at least one of them. And moreover that one happens to be isolated from other material. [2]

[1] https://en.m.wikipedia.org/wiki/List_of_nearest_known_black_...

[2] https://en.m.wikipedia.org/wiki/OGLE-2011-BLG-0462

Fermi problem: How many photons do? Have you or I ever seen one?

wait until we launch the next telescope... er graviscope

Is there anything we can do deliberately to use gravitational lensing other than aiming a telescope at a part of the sky where this is happening by random chance?

Video discussing the use of the sun as a gravitational lens to image exoplanets: https://youtu.be/NQFqDKRAROI

Other than deliberately pointing a telescope at massive objects, I'd say no.

You would probably need a telescope larger than earth to do this.

Using interferometery of orbiting observatories (potentially on solar orbits, to leverage the entire diameter of the solar system), this is conceivably possible

Having just visited Mt. Wilson, and learning about CHARA, this idea sounds fantastic.

Would there be fotons of walking dinosaur still floating around into our direction after making a complete u turn somewhere?

Milky Way core is approx 25k ly away so you’d see earth 50k years ago. Sorry no Dinos.

i read somewhere recently that this difference in path lengths can be measured in days, depending on distance and how off-center the lensing is.

That's pretty amazing, I'd certainly not expect more than a small time shift since it still managing to get the same light source to the same location to be detected.

The lensed light has generally traveled for billions of years, and wound up being separated by thousands of light years before the gravitational lensing. Two paths taking days apart DOES count as only a small time shift!

or decades

https://news.berkeley.edu/2015/03/05/distant-supernova-split...

I seem to remember an astronomer mentioning that it is possible to interfere the light from gravitationally lensed double images. Lacking appropriate intuition, this would be amazing for me as I’d think the photons would lose coherence over the course of their multi billion light year journeys (but maybe not?). Does anyone know about this?

Interferometry is common in radio telescopes, and was the method used in the recent imaging of a black hole by the Event Horizon Telescope, which used interferometry between multiple telescopes on different sides of the earth. It's also used a bit in optical telescopes, but harder to do. Basically yes, interference can still be used even over these extreme distances. The speed of light is pretty exact I suppose!

https://en.m.wikipedia.org/wiki/Event_Horizon_Telescope

https://en.m.wikipedia.org/wiki/Astronomical_interferometer

Interesting, thanks for the links. Looking at them, though, it would appear that the two "paths" being interfered differ in length on the order of the separation between the two telescopes -- i.e. meters or kilometers -- rather than astronomical distances. Using the speed of light, this corresponds to a difference in flight time of the light on the order of nanoseconds or seconds. I can imagine how the light traversing the two paths could remain unmolested by stray random, environmental phase shifts during such short times (necessary if you want them to interfere). On the other hand, the double images in the photo would have a difference in flight times of years? But, again, I don't really understand precisely what the astronomer was referring to, or have proper intuition for the conditions of space through which astronomical photons travel...

Photons travel at c, so don’t they experience no time? Wouldn’t they be zero old?

True that, but from the point of view of an outside observer, their wavefunction does have a phase that regularly oscillates (with a frequency corresponding to the color of the photon)

That has always been my take. It explains how they always perform to spec even across unimaginable distance and time.

MTTF at infinity due to relativistic effects is a pretty funny idea.

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